In order to analyze the characteristics of high frequency components such as communication devices (device under test) used in various communication systems, a network analyzer is frequently used. A network analyzer obtains various test parameters, such as a transfer function, reflection characteristics, and phase characteristics (hereafter “scattering parameter S” or “S-parameter”), of a device under test. Such S-parameters are known in the art and determined by observing the frequency response (voltage and phase) of the device under test resulted in response to a test signal from the network analyzer.
A network analyzer is usually comprised of two ports, one is an input port and the other one is an output port. The input port sends a sweep frequency signal (test signal) to the device under test and the output port receives the response output signal of the device under test. The input port and the output port of the network analyzer are usually organized such that either port can be switched to the other by a switching operation in the network analyzer. An example of configuration of such a network analyzer is shown in a block diagram of FIG. 1.
The configuration and operation of the network analyzer shown in FIG. 1 is briefly explained. A network analyzer 10 has two input-output ports (test ports) P1 and P2 which are connected to directional bridges (or directional couplers) 51 and 52, respectively. Each of the bridges 51 and 52 functions as a signal separation circuit. A test signal from a signal generator 55 is sent to one of either the bridge 51 or bridge 52 which is selected by a switch 53. The test signal (sweep frequency signal) is sent from the selected one of the test port P1 or test port P2 to a device under test (not shown). The test signal from the signal generator 55 is also sent to the inside of the network analyzer as a reference signal. Namely, this reference signal and the input signal from the bridge 51 or 52 are respectively provided to frequency converters 57, 58 and 59 whereby converted to signals of a lower frequency.
The frequency converted input signal and the reference signal are respectively converted to digital signals by AD converters 61, 62 and 63. The digital signals are processed by a digital signal processor (DSP) 65 to determine S-parameters of the device under test. The S-parameters or other data derived from the S-parameters are displayed by a display 69 in various formats under the control of a CPU 68 which controls the overall operation of the system.
The devices to be tested, for example, devices and components such as used in communication systems, are sometimes formed with not only two terminals (ports) but also three or more terminals (hereinafter may also be referred to as “multi-port device”). In order to measure the S-parameters of the multi-port devices, an S-parameter test set having three or more ports may be used in combination with the network analyzer having two ports. Such an example is shown in FIG. 2 wherein a three port DUT is connected to a three port S-parameter test set having three ports.
In using the three port test set of FIG. 2, before connecting the DUT to test ports 90, 92 and 94, the test set is preferably calibrated to test the DUT with high accuracy. Typically, such a calibration process is conducted by using a predetermined two port calibration set between the test ports 90 and 92, between the test ports 92 and 94, and between the test ports 94 and 92. Then the DUT is connected to the test set and the S-parameters are measured.
The process for measuring the S-parameters of three port device with use of the convention network analyzer is described in more detail. FIG. 3 is block diagram showing an example of network analyzer designed for three port device testing. The network analyzer 200 of FIG. 3 includes a three port test set therein, and thus functions in the same manner as the example of FIG. 2.
The network analyzer 200 includes a signal source 210 which generates a sweep frequency signal, switches 212, 214, 216, 218 and 220, each having two switching circuits (designated by circle 1 and circle 2), a receiver circuit 222 and three direction bridges (couplers) 230, 232 and 234. The receiver circuit 222 includes three measurement units 224, 226 and 228. The receiver circuit 222 of FIG. 3 thus corresponds to the frequency converters 57, 58, 59 and the A/D converters 61, 62, 63 and the DSP 65 of FIG. 1. The measurement unit 228 is to measure a signal level of the signal source 210, i.e., a reference level “R”. The other measurement units 224 and 226 are to measure signal levels of output signals (transmission signal and/or reflection signal) from the device under test (DUT 40). In this example, measured results based on the voltage ratio between the measurement units 224 and 228 is denoted as “measurement A” and measured results based on the voltage ratio between the measurement units 226 and 228 is denoted as “measurement B”.
FIG. 4 is a table showing a relationship between types of S-parameters and switch settings and number of signal sweep operation when testing the S-parameters of the three port device 300 by the network analyzer of FIG. 3. In FIG. 4, labels SW1-SW5 correspond to the switches 212-220 of FIG. 3, respectively. When the switching circuit (circle 1 or circle 2) in the switch is ON, it is connected to a path to other circuit components, and when the switching circuit is OFF, it is connected to the ground through a terminal resistor.
The three port device (DUT) 40 is connected to test ports 240, 242 and 244 of the network analyzer 200. First, the switch setting is made so that the test signal is provided to the DUT 40 through the test port 240. Under this condition, the network analyzer 200 measures S-parameters S11, S21 and S31 of the DUT 40. For example, for measuring S-parameter S11, the test (sweep frequency) signal 210 is supplied to the DUT 40 through the switch 212 (SW1) and the test port 240. At the same time, a reflected signal from an input terminal (1) of the DUT 40 is received by the measurement unit 224 through the directional bridge 230 and the switch 216 (SW3) to conduct the “measurement A”. Also at the same time, for measuring S-parameter S21, a transmission signal from a terminal (2) of the DUT 40 is received by the measurement unit 226 through the bridge 232 and the switches 218 (SW4) and 220 (SW5) to conduct the “measurement B”. Thus, S-parameters S11 and S21 can be measured by a single sweep of the test signal 210.
For measuring S-parameter S31, while applying the test signal 210 to the terminal (1) of the DUT 40 through the test port 240, a transmission signal from the terminal (3) of the DUT 40 is measured. Thus, the switch 5 is changed its connection so that the transmission signal from the terminal (3) of the DUT 40 is received by the measurement unit 226 through the directional bridge 234 and the switch 220. As in the foregoing, for measuring S-parameters S11, S21 and S31, the sweep signal must be applied to the terminal (1) by two times as shown in the left column of FIG. 4.
In a similar manner, by applying the test signal to the terminal (2) of the DUT 40, the network analyzer 200 measures S-parameters S12, S22 and S32 of the DUT 40 under the settings shown in the center column of FIG. 4. The network analyzer 200 further measures S-parameters S13, S23 and S33 of the DUT 40 under the settings shown in the right column of FIG. 4. Thus, all of the S-parameters are measured in the forgoing procedure and conditions.
In the measurement by the three port test set of FIG. 2 or the three port network analyzer 200 of FIG. 3, however, there is a problem in that the measurement accuracy of a three port device under test is not high enough even after conducting the calibration procedure between two test ports (two port calibration). More specifically, two port calibration will be conducted between the test ports 90 and 92 in FIG. 2 (240 and 242 in FIG. 3), the test ports 92 and 94 in FIG. 2 (242 and 244 in FIG. 3), and the test ports 94 and 90 in FIG. 2 (244 and 240 in FIG. 3) before testing the DUT. However, by the calibration procedure above, although error items (coefficients) between the two test ports can be removed, error coefficients in the third test port are not fully calibrated. For example, in the calibration between the test ports 90 and 92 (240 and 242), the error at the test port 94 under the situation is not measured or compensated.
Other problem involved in measuring the S-parameters by the conventional test set or network analyzer 200 as noted above is that it requires a considerably long time to complete the measurement. For example, as shown in the table of FIG. 4, for measuring each set of three S-parameters, the sweep test signal must be applied to the DUT by two times. Thus, for obtaining all of the nine S-parameters, the test signal sweep must be repeatedly applied to the DUT by six times, resulting in a long time before completing the measurement.
A further problem is directed to a signal loss, i.e., a measurement dynamic range. Since the example of FIG. 3 includes the switches 218 which is series connected to the switch 216 or switch 220 for transmitting the signal from the DUT, a signal loss will be incurred before the signal reaching the measurement units 224 or 226. Such a signal loss decreases a measurement dynamic range or measurement sensitivity in the network analyzer.
In testing the three port device (DUT) by a two port network analyzer (FIG. 5A) or through a two port test set (FIG. 5B), the third terminal of the DUT must be terminated through a terminal resistor of known value. Before the S-parameter measurement, the two port calibration is performed between the two test ports P1 and P2 (Q1 and Q2). Then, two ports (terminals) of the DUT are connected to the test ports of the network analyzer (FIG. 5A) or the test set (FIG. 5B) while the remaining port of the DUT is connected to a terminal resistor R. Under this condition, S-parameters of the two ports of the DUT are measured. Then, by connecting the next two ports of the DUT to the test ports and connecting the terminal resistor R to the remaining port of the DUT, S-parameters concerning the connected two ports are measured. By repeating the similar process by one more time, all of S-parameters can be obtained.
In the measurement by using the two port network analyzer of FIG. 5A or the two port test set of FIG. 5B noted above, connections between the DUT and the network analyzer (test set) and terminal resistor R have to be manually changed many times. Therefore, this test method is disadvantageous in that it is complicated and time consuming. Moreover, in the case where the terminal resistor R is deviated from the ideal value, a reflection at the port of the terminal resistor R may occur, resulting in errors in the measurement of the S-parameters.